The EU's cyber security Agency, ENISA, wants a re-think of how we measure the size and potency of botnets, networks of malware-infected PCs that are now the mainstay of spam distribution, identity theft and DDoS (distributed denial of service) attacks.

Two parallel studies by ENISA, both due to be published at a security conference in Cologne on Wednesday, collectively evaluate the botnet threat while assessing the effectiveness of possible countermeasures and making recommendations to EU governments.

One of the two ENISA reports, entitled Botnets: 10 Tough Questions and based on interviews with cybercrime experts including security researchers and law enforcement officials, questions the reliability of botnet size estimates.

The study also looks the role of government in the fight against cybercrime as well as evaluating where money might be better spent.

Getting an accurate idea of the scope of the botnet threat is important because it informs how millions of information security-related dollars and pounds are spent.

Dr Giles Hogben, the report editor, concludes that current measures of the botnet threat (which are largely based on the numbers of drones in a particular zombie-machine network) could be misleading. "Size is not everything – the number of infected machines alone is an inappropriate measure of the threat" says Hogben.

A second study, Botnets: Measurement, Detection, Disinfection and Defence, evaluates best practice methods in combating zombie networks as well as providing an assessment of the threat posed by botnets. Recommendations in the report cover the legal, policy and technical aspects of the fight against botnets. In particular, ENISA backs schemes involving the notification of infected customers by their ISPs, a practice so far restricted to trial schemes in a small number of countries, including Australia. The survey also advocates closer international cooperation between governments and industry in the fight against cyber zombies.

ENISA has looked at the botnet threat before, with a study published three years ago which also emphasised international co-operation and the sharing of best practice. Three years later, ENISA is placing greater emphasis on scoping out the scale of the zombie network problem and on infection notification by ISPs than its previous study, which was published in May 2008.

More on ENISA's latest thinking on the pervasive cybercrime threat from infected networks of zombie PCs can be found on the agency's microsite here: http://www.enisa.europa.eu/botnets